Flip chip bonding is known as one of the assembly technology of a semiconductor device. This bonding method is a method which sticks the connection pad (only henceforth a “pad”) formed in the under surface of the semiconductor chip, and the pad formed in the package substrate upper surface, applies heat and pressure, and is joined. On each pad of a semiconductor chip and a package substrate, the solder bump of ball state is beforehand formed in the case. Usually, the solder bump is put in order in a lattice manner, and such a structure is called BGA (Ball Grid Array) structure.
Conventionally, Sn(tin)—Pb (lead) eutectic solder was used as solder used for a solder bump. However, in recent years, in order to inhibit the bad influence to the environment at the time of doing disposal treatment of the electronic parts, the so-called lead free solder that does not include Pb as a solder alloy is used widely.
As lead free solder used for a solder bump, the solder alloy of the so-called “Sn—Ag—Cu system” which comprises Sn, Ag (silver), and Cu (copper) is known widely. The solder alloy which consists of Ag:3-4 mass %, Cu:0.5-1.0 mass %, and Sn as the remainder especially was common. However, the solder alloy for solder bumps which did not use Ag expensive in material price so much (below 2 mass %), but was excellent in joining reliability and dropping impact-proof nature is also proposed (for example, following Patent Reference 1).
In connection with the microfabrication of a semiconductor chip, the pad size of the semiconductor chip is also small. Therefore, a solder bump's volume and the junction area of a solder bump and a pad will become small inevitably, and the strength of a junction portion will fall. Usually, in order to compensate it, both gap is filled up with under-filling resin, such as an epoxy resin, after doing bonding of the semiconductor chip to a package substrate. Under-filling resin pastes up between a semiconductor chip and a package substrate, and it makes the external force concerning a solder bump's junction ease.
On the other hand, the electrode for external connection is formed in the under surface of the package substrate of a semiconductor device. The solder ball used when mounted in mounting substrates, such as a mother board of a computer, on it is formed. And when mounted, the semiconductor device concerned is soldered to a mounting substrate by mounting a semiconductor device on a mounting substrate and heating (reflowing) it so that a solder ball may contact the connection pad of a mounting substrate.
Usually, so that it may not melt with heating of an assembling step, as for the solder bump as internal wiring which connects a semiconductor chip and a package substrate, what has a melting point higher than the solder ball as external wiring under a package substrate is used. By using what has a small ratio over the volume at the time of solid of the volume difference of a volume at the time of solid and a volume at the time of melting as a solder bump's material, even when it melts with heating at the time of mounting, there is also technology of preventing the short circuit between solder bumps (for example, following Patent Reference 2).
For convenience of explanation, the solder bump as internal wiring which connects a package substrate with a semiconductor chip may be hereafter called an “inner bump”, and the solder ball as external wiring which connects a package substrate with the outside (mother board etc.) may be called an “outer ball”.    [Patent Reference 1] Japanese Unexamined Patent Publication No. 2002-239780    [Patent Reference 2] Japanese Unexamined Patent Publication No. 2004-207494
Although the coefficient of thermal expansion of the semiconductor chip of common silicon is about 7 ppm/° C. here, the package substrate and mounting substrate of resin are about 20 ppm/° C., and under-filling resin of an epoxy resin is about 30 ppm/° C., solder is about 15 ppm/° C. and they differ from each other, respectively. Therefore, by a temperature change, internal stress occurs in a part between the semiconductor chips and package substrates inside a semiconductor device, and for the connecting part between the package substrate and mounting substrate at the time of mounting. The stress was applied to the inner bump or the outer ball, became a factor which causes disconnection, and was reducing the connection reliability of the semiconductor device.